Artificial teeth may be fashioned from porcelain which is baked onto a metal substrate. The metal substrate is subsequently used for anchoring the artificial teeth in the person's mouth either on a temporary or permanent basis. The goal of the dental technician is to provide as realistic looking an artificial tooth as possible and therefore the technology involved in this branch of the dental art relates to the appearance of the tooth. Of course, the dental technician must also consider the functional nature of the tooth, i.e. as a chewing or biting surface.
The metal substrate such as a noble metal, i.e. gold; or a non precious metal, i.e. the various nickel and chromium containing alloys which have been recently introduced to the market; is treated to prepare the surface for the coating of the porcelain thereon. The gold substrate may be partially oxidized, for example, to make it easier to bond porcelain thereto. The non noble metal alloys may be treated with a bonding agent, e.g. a dispersion of aluminum and boron powder in a vehicle such as propylene glycol, to prepare the surface for bonding to the porcelain.
The dental technician, after preparing the metal surface, will paint a dispersion of p-o-p (paint opaque) porcelain onto the metal and may fire the dispersion coated metal by heating over a temperature of from 1200.degree. F. to 1700.degree. F. in vacuum, at a rate increase of 90.degree. to 100.degree. F. per minute. After a temperature of 1700.degree. F. is reached the vacuum may be broken and the coated metal substrate heated in air at the same temperature rate increase up to 1900.degree. F. The p-o-p porcelain is opaque and hides the metal surface of the artificial tooth completely.
This first porcelain coating is followed by a second coating of a more translucent porcelain known as gingival porcelain. The gingival porcelain is also painted as a dispersion onto the previously fired, porcelain coated metal substrate but before firing a third porcelain known as the incisal porcelain is used to coat that portion of the coated substrate which will form the incisal surface of the artificial tooth. The incisal porcelain is even more translucent than the gingival procelain, thus assuring the realistic look of the artificial tooth. Both the gingival and incisal porcelain may have coloring agents added to match the color of the natural teeth of the person who is to use the artificial tooth or teeth. The incisal porcelain will generally be coated from the incisal edge approximately a third of the way up toward the gingival surface.
After the gingival and incisal porcelain are coated onto the p-o-p porcelain coated metal substrate the tooth is fired in a manner similar to that of the p-o-p porcelain.
If the dental technician has been completely accurate the artificial tooth may be ready at this point for the wearer, however, it generally requies further treatment to fit and look presentable. The dental technician may, for example, have to grind down portions of the tooth so that it will fit more correctly into the mouth of the wearer. The grinding of course, removes any glossy appearance from the surface of the artificial tooth, therefore, a further coating of gingival and/or incisal porcelain, from a dispersion, will be required and the further coated tooth will be fired in a manner similar to the above. As can be appreciated there may be many such further coatings and refirings. Generally, the dental technician, if proficient in his skill, will take approximately three such grindings, coatings and refirings to prepare the artificial tooth. However, up to seven is not uncommon, especially when the dental technician is not of the highest skill.
It is known in the art of ceramics that there is a vitrification point in which the ceramic material has its greatest density. From the standpoint of strength, it is desirable that the material be at the vitrification point. Generally, dental ceramics such as the porcelain described above, vitrify at about 1800.degree. to 1850.degree. F. Therefore, it is evident why the above temperature-time cycle is used to prepare the artificial tooth. It has been found, however, that if you refire many times the vitrification point will be exceeded and it will be found that the ceramic is no longer at its greatest density. The dental technician may notice that the structure will slump after repeated firings thus changing both the shape and the strength of the artificial tooth. Repeated firings also may give rise to an additional problem known as `checking`. The artificial tooth becomes crazed with spider weblike cracks, again detracting from a natural appearance.
It is also known in the ceramic art, especially the dental ceramic art, that the surface area of the artificial tooth should be somewhat glossy to resemble the appearance of a natural tooth. The above dental ceramics are somewhat glossy after heating to temperatures of about 1800.degree. F. and above, that is they are self glazing when heated to 1800.degree. F. Therefore, subsequent grindings remove the glossy surface and require reheating the artificial tooth to temperatures of at least 1800.degree. F. to restore the gloss. Repeated heatings, of course, as noted above, cause the tooth to pass the vitrification point and then it is no longer suited for its intended end use.
It will thus be clear that repeated coating, grinding and refiring may result in having artificial teeth that are no longer suitable for their end use.
Various products have been put onto the market which are touted as useful for glazing artificial teeth. These products claim to allow the dental technician to heat to a lower temperature after grinding the tooth and obtain a glazed, naturally looking surface. The obvious advantage to products of this kind is evident, in that the dental technician would not have to take the risk of heating the artificial tooth to a point where it is self glazing and thus risk exceeding the vitrification point. It has been found, however, that the glazes on the market mature, that is, form the glossy surface, at temperatures starting at about 1700.degree. F. up to and including 1800.degree. F. Thus to obtain the optimal properties of these glazes the dental technician has to heat to temperatures as high as the temperature at which the dental porcelain is self glazing. To avoid the risk of slumping and checking, the dental technician usually underfires the artificial tooth coated with these prior art glazes, that is, he may heat to a temperature of no more than 1700.degree. F. At this temperature, although he may improve the surface appearance of the artificial tooth as compared to a ground surface, he will not achieve an appearance equivalent to a self glazed ceramic or a natural tooth. Usually a somewhat gritty surface is obtained if the glaze is underfired. Therefore, the use of the prior art glazes represent, at best, a compromise. A dental ceramic glaze which matures at a lower temperature thus would be very valuable to the dental technician.
Various glazes which are purported to mature at a low temperature have been reported in the literature. See, for example, Bulletin, "Dental Porcelain" No. 118 of the Ohio State University Engineering Experiment Station, March 1944. The glazes disclosed therein differ from the instant novel glazes in chemical composition. Moreover, there is no teaching of a preferred average particle size limitation which enables the dental technician to prepare dispersions which are easily painted onto porcelain, coated metal substrate.